1. Field of the Invention
The present invention relates to a semiconductor substrate used in a power semiconductor device and a method of manufacturing the same, and a semiconductor device using the semiconductor substrate and a method of manufacturing the same.
2. Description of the Background Art
In recent, a power circuit called an AC matrix converter in which direct switching of a three-phase voltage source is performed by a bidirectional switch has been proposed. As the bidirectional switch used in the AC matrix converter, a power device having bidirectional breakdown voltages is required. As an example of this, an IGBT capable of bidirectionally retaining its breakdown voltages is disclosed in M. Takei, Y. Harada, and K. Ueno, “600V-IGBT with Reverse Blocking Capability”, Proceedings of 2001 International Symposium on Power Semiconductor Devices & ICs, Osaka.
Also, a technique for forming a local lifetime region by helium irradiation or proton irradiation is disclosed in Japanese Patent Application Laid-Open No. 2002-76017.
In the IGBT described in Takei et al., however, by providing a trench of mesa structure from a surface of a substrate to a collector P layer and forming a substance to relieve an electric field inside the trench, the breakdown voltage is retained. Though this method is adopted in an existing triac and the like, it has a problem of low reliability.
In addition, in the above-mentioned Japanese Patent Application Laid-Open No. 2002-76017, helium and proton are treated alike, but, depending on the implantation depth of proton into the substrate, a problem arises that reverse breakdown voltage reduces because the implanted protons are made to act as a donor.